Defense: Sign-off for conelrad

Back in 1951, when the U.S. began to worry about Russian-atom armed
bombers, somebody had a notion that the invaders might steer by the
crisscrossing waves of U.S. commercial broadcasting stations. Probably
Russian navigators were never so helpless as that, but an official
system, Conelrad (for Control of Electromagnetic Radiation), was set up
to foil them. Under Conelrad regulations, all regular broadcasting
would go silent during an attack, while stations going on and off the
air on two special frequencies, 640 and 1240 kc., would stand ready to
give instructions and comfort to the quaking population.